Light emitting diodes (LEDs) emitting white light are next-generation light emitting device candidates which can replace fluorescent lights as the most representative conventional lights.
Light emitting diodes have low power consumption as compared to conventional light sources and are environmentally friendly because they do not contain mercury, unlike fluorescent lights. In addition, light emitting diodes have advantages of long lifespan and high response speed as compared to conventional light sources.
Such an LED may be used in combination with a phosphor which absorbs light emitted from the LED and emits various colors of light. The phosphor generally emits white, green and red light.
Currently, white LEDs are produced using a combination of blue LEDs and phosphors for changing emission wavelengths. As the field of white LEDs used extends, there is a need for more efficient LEDs. For this purpose, there is a need for improved luminous efficacy of phosphors. In addition, there is an increasing demand for LEDs with improved reliability.
Phosphors used for LEDs are yellow phosphors and YAG phosphors are known as oxide phosphors. However, such YAG phosphors have problems of low thermal stability, deterioration in brightness when heated, variation in color coordinates and the like.
In addition, oxide phosphors and silicate phosphors are known as yellow to green phosphors, but they have low thermal stability and poor resistance to humidity, thus negatively affecting reliability of LED packages.
Meanwhile, Prior art Patent Document 1 (Japanese Patent Laid-open No. 3851331) discloses an oxynitride phosphor activated by a rare-earth element, wherein the oxynitride phosphor is represented by MSi2N2O2 (in which M includes one or more alkali earth metal elements).
In addition, Prior art Patent Document 2 (Japanese Patent Laid-open No. 4442101) discloses an oxynitride phosphor represented by SrSi2N2O2:Eu. This discloses that green to yellow light emission phosphors which are excited by a ultraviolet to visible range of excitation light sources.
However, these oxynitride phosphors have excellent thermal stability and eco-friendliness, as compared to oxide phosphors, but lack brightness and require improvement in luminous efficacy.